Field of the Invention
The present invention relates tires to be mounted to vehicles.
Description of the Related Art
A tire includes a tread, sidewalls, beads, and the like. Each bead includes a core, and an apex that extends from the core in the radial direction. The apex contributes to improvement of stiffness and durability of the tire. In general, from the viewpoint of improvement of stiffness and durability, the height of the apex is increased to some degree. In particular, in a tire that is to be under heavy load, the height of the apex is increased.
When the vehicle runs, the tire rolls on a road surface. The ground contact surface of the tire varies in the circumferential direction. Deformed portions in the tread, the sidewalls, the carcass, and the like of the tire vary in the circumferential direction. Deformation of each portion of the tire is periodically repeated due to varying of the deformed portions. In the sidewalls and the beads, load applied around portions that contact the ground, and load applied to the other portions are different. The periodic deformation causes increase of distortion and heat generation in the tire. The increase of distortion and heat generation causes reduction of durability of the tire. Further, in a tire in which the height of the apex is increased, damage such as separation may occur around the apex, and durability of the tire is likely to be reduced.
In JP2013-545671 (US2014/0000780) and WO2012/18106 (US2013/0133806), tires are disclosed which include: apexes each having a reduced height; and fillers and the like provided outward of the apexes in the axial direction. In these tires, deformation of the apexes can be reduced.
Also in these tires, when load thereon is heavy, great deformation of the leading end portion of the apex is repeated. In particular, near a radially outer edge at which the tire and a flange of a rim contact each other, the deformation is likely to be increased, and distortion and heat generation may be increased. Also in these tires, enhanced improvement is required in view of durability. In particular, for the tire which is to be under heavy load, enhanced improvement of durability is required. An object of the present invention is to provide a tire excellent in durability.
The apex is formed of a highly hard crosslinked rubber. The apex contributes to improvement of stiffness of the tire. In a case where the highly hard apex is left in a certain deformed state for a long time period, permanent set may occur. In the tire in which the height of the apex is increased, the permanent set may cause deformation of the sidewalls and the tread. The deformation of the tread is referred to as a so-called flat spot. In order to reduce generation of the flat spot, the height of the apex is advantageously reduced.
Tires in which the height of apexes is reduced are disclosed in, for example, JP2013-169825, JP2013-545671 (US2014/0000780), and WO2012/18106 (US2013/0133806). Reduction of the height of the apex allows generation of the flat spot to be reduced.
Meanwhile, when the height of the apex is excessively reduced, or the apex is excessively thin, durability or handling stability of the tire may be deteriorated. The height and the thickness of the apex exert influence also on durability and handling stability of the tire. Some prior art documents disclose tires in which fillers or the like are disposed outward of the apexes in the axial direction. In these tires, deterioration of durability or handling stability of the tire is reduced by using apexes and fillers or the like in combination.
However, it is not easy to simultaneously achieve reduction of generation of a flat spot, improvement of durability, and improvement of handling stability even in these tires. Another object of the present invention is to provide a tire that is excellent in durability and handling stability and simultaneously allows generation of a flat spot to be reduced.